Sorbent based gas concentration monitor

ABSTRACT

A gas monitor apparatus includes a sorbent material that adsorbs a target gas based on a concentration of the target gas in a monitored environment and a reference material that does not respond to the target gas. The gas monitor also includes a first thermistor disposed within the sorbent material and a second thermistor disposed within the reference material, the first thermistor to provide a first indication of a first temperature of the sorbent material and the second thermistor to provide a second indication of a second temperature of the reference material. A processing device determines a concentration of the target gas based at least in part on a differential measurement between the first temperature and the second temperature.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/800,788, filed Nov. 1, 2017, which is hereby incorporated byreference.

TECHNICAL FIELD

Implementations of the present disclosure relate to measurement ofgasses present in a system.

BACKGROUND

Gasses present in a system of atmosphere may affect people or theperformance of components present in the area. Measurement of gasses maybe performed with a number of techniques based on properties of the gasand the concentration of the gas present.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and the advantages thereof may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings. These drawings in no waylimit any changes in form and detail that may be made to the describedembodiments by one skilled in the art without departing from the spiritand scope of the described embodiments.

FIG. 1 is a schematic diagram of an embodiment of a gas monitor, whichcan be used in accordance with some embodiments.

FIG. 2 is a schematic diagram of an embodiment of a gas monitor, whichcan be used in accordance with some embodiments.

FIG. 3A is a graph showing an example relationship between the quantityof a target gas adsorbed by a sorbent and the change in the heat ofadsorption for the system.

FIG. 3B is a graph showing an example relationship between theconcentration of the target gas in the monitored environment and theamount of the target gas on a carbon sorbent.

FIG. 4 is a graph showing an example of a differential temperatureresponse of a sorbent material and reference material when exposed tochanging CO₂ concentrations.

FIG. 5 is a graph showing an example of a differential temperatureresponse of a sorbent material and reference material when exposed tochanging CO₂ concentrations, according to an embodiment.

FIG. 6 is a graph showing an example of adsorptive loading on a sorbentas a function of temperature and gas concentration.

FIG. 7 is a flow diagram depicting a method of determining concentrationof a target gas based on the temperature of a sorbent material, inaccordance with some embodiments.

FIG. 8 is a flow diagram depicting a method of determining concentrationof a target gas based on the temperature of a sorbent material, inaccordance with some embodiments.

FIG. 9 is a flow diagram depicting a method of manufacturing a gasmonitor, in accordance with some embodiments.

FIG. 10 is a schematic diagram of an embodiment of gas monitoringsystem, which can be used in accordance with some embodiments.

DETAILED DESCRIPTION

Measurement of gas levels within different environments is important toensure quality of air, lack of pollutants, quality control inmanufacturing, and a number of different reasons. However, some gaslevel monitoring solutions may be expensive, large, require high powerconsumption, or have other drawbacks that prevent widespread use withindifferent environments. In addition, with increasing connection betweenvarious consumer devices, opportunities for remote sensing of varioushome parameters exist. For example Internet of Things connected devicesmay be dispersed through a home or facility and provide information toone another, to a central server, or to a local control device. Large,expensive, or high power consuming gas monitors may not be practical forimplementation in a connected facility. Accordingly, smaller, cheaper,and low power consumption gas monitoring systems may enable widerutilization of such measurement systems.

In some embodiments, a gas monitor monitors the temperature of a sorbentmaterial to determine changes to the concentration of a gas within asystem. For example, if the concentration of a gas in a monitoredenvironment is increased, the sorbent material may adsorb more of thegas. Then, as the gas is adsorbed by the sorbent material, thetemperature of the sorbent may increase. Thus, the gas monitordetermines a concentration of the gas, or a change in the concentrationof the gas, based on monitoring the temperature of the sorbent material.

In some embodiments, a gas monitor may include a reference material inaddition to the sorbent material. The reference material may act as acontrol to compare with the gas monitor. The gas monitor may determine adifferential measurement between the sorbent material and the referencematerial. The differential measurement may provide an indication of thechange in the temperature of the sorbent material compared to changes inthe environment due to other causes. For example, due to changes in airtemperature of the monitored environment.

In some embodiments, the gas monitor may passively measure changes tothe differential temperature of the sorbent material and referencematerial. If the concentration of a target gas increases, then thesorbent material may adsorb more of the gas and increase temperature.Thus, the differential measurement may increase, indicating that thetemperature of the sorbent material increased. If the concentration ofthe target gas decreases, then the sorbent material may release thetarget gas back to the atmosphere and decrease in temperature. Thus, thedifferential measurement may also decrease, indicating that thetemperature of the sorbent material decreased. Based on the changes inthe differential temperature measurement, the gas monitor may determineif the concentration of the target gas changes. The gas monitor mayprovide an indication or report of any change of the concentration ofthe target gas to a monitoring or alarm system.

Measuring the differential temperature of the sorbent material and thereference material may provide an indication of the changes to theconcentration of the target gas. However, in some situations, it may bebeneficial for the gas monitor to determine an absolute value for theconcentration of the gas in the monitored environment. Therefore, insome embodiments, the temperature of the sorbent material and referencematerial may be controlled to monitor absolute characteristics of theenvironment. For example, by raising and lowering the temperature of thesorbent material, the sorbent material may respectively adsorb andrelease a target gas. Furthermore, in some embodiments, a gas monitormay calibrate the system by heating the system to desorb all of thetarget gas from the sorbent. Adsorbing and releasing the target gas mayaffect the rate of temperature change of the sorbent material comparedto the reference material. The gas monitor may determine a differentialtemperature measurement between the sorbent material and the referencematerial based on the difference in the rate of change of thetemperature of the sorbent material. The change in the rate of change ofthe temperature of the sorbent material is based on the concentration ofthe target gas in the monitored environment. Accordingly, the gasmonitor may determine an absolute measurement for concentration of thetarget gas in the monitored environment based on the differentialmeasurement as the temperature of the sorbent material and the referencematerial are raised and lowered by the gas monitor.

In order to provide an accurate measurement of changing concentrationsof a target gas or absolute concentrations of a target gas, appropriatesorbent materials and reference materials may be selected. In someembodiments, the sorbent material may be reasonably selective to aparticular target gas or class of target gasses. For example, thesorbent may only adsorb a particular gas or a set of gasses that arepart of a family of related gasses. A sorbent may also be placed withina system having filters or getters to improve selectivity. For example,filters and getters may prevent the sorbent from adsorbing gasses otherthan the target gas. This may increase the relative selectivity of thesorbent for the target gas. In some embodiments, the sorbent materialmay also have a high thermal conductivity so that heat is transferred toa thermistor or other temperature measurement device. In someembodiments, the high thermal conductivity may also provide bettertemperature control by a heat source. The sorbent material may also havea high surface area, or may be a porous structure, in order to improvethe amount of target gas loading on the sorbent. Additionally, thesorbent material may have an intermediate binding energy. For example,the intermediate binding energy may provide a reversible interactionwith the target gas (i.e., adsorption and desorption). The reversibleinteractions may provide the gas monitor with the ability to monitor theabsolute concentration of the target gas over time.

In addition to the sorbent material, a reference material may beselected that has similar thermal properties to the sorbent material.Having similar thermal properties may provide an accurate differentialmeasurement between the reference material and the sorbent material. Forexample, if the reference material had lower thermal conductivity, adifferential measurement may be formed based on the difference betweenthe rates of change to an ambient temperature of the environment andreduce the accuracy of the gas monitor.

The gas monitors described herein may be utilized for any gasmeasurement that has a sorbent material suitably selected to adsorb andrelease a target gas and generate a reasonable heat of adsorption. Forexample, in some embodiments, the gas monitor may selectively adsorbCO₂. CO₂ monitoring may be important for maintaining healthy indoor airquality. Additionally, there are various regulations indicating CO₂levels that are appropriate for occupied indoor spaces. While varioustechniques may be applied to generate ventilation requirements to meetstandards and regulations, without measuring CO₂ levels in particularrooms of a building, some parts of the building may be over ventilatedor under ventilated. Accordingly, CO₂ levels may not meet standards inall areas, or excessive energy may be expended to over ventilate someareas. To improve air quality and energy costs, deploying multiple CO2monitors as described herein may give additional insight into the actualCO₂ levels of rooms. As an example, ASHRAE Standard 62.1-2016 indicatesthat concentrations of CO₂ in an indoor space should be kept below 700ppm above outdoor air concentration levels. It is suggested thatapproximately 7.5 L/s/person of ventilation is required, but this is nota direct indication of the actual concentration of CO₂ in indoor air.The actual ventilation required to achieve the standard could be basedon activity level and other features of a particular indoor space. Thus,by reducing the size and power consumption of devices to enable morewidespread deployment of CO₂ sensors as described herein, better airquality may be achieved at a lower energy consumption.

In some embodiments, an appropriate binder may effectively improve thethermal conductivity of the sorbent material and the reference material.Notably, with a high surface area porous sorbent, a large molecular size(for example, greater than 0.1 μm) binder may reduce the chance thatsorbent particles are filled with the binder. In some embodiments, acarbon sorbent may be used for detection of CO₂. For example, EntegrisBrightBlack® may be used as a sorbent. In some embodiments, other carbonsorbents may be used instead of Entegris BrightBlack®. For example,another microporous and nanoporous carbon material may be used. In someembodiments, other high surface materials with medium binding energysurface groups may be used. For example, the sorbent may be ametal-organic-framework, a zeolite, carbon nanotubes, graphenes,silanized aerogels, or a combination of such materials. In an examplebinder related to a carbon sorbent of CO₂ gas, a styrene acrylic-basedpolymer latex with a molecular size of about 0.1-0.15 μm or porousglassy solid binder from colloidal silica may provide a suitablestructure. In some embodiments, a carbon sorbent used in the sorbentmaterial may act as a molecular sieve, with pore size below a fewnanometers. Thus, pore filling may be limited with a number of binders.However, surface blocking still needs to be minimized to enhance thekinetics of the target gas adsorption into the sorbent material.Accordingly, dispersion of the carbon sorbent particles and ahomogeneous distribution of particles and binders may improve thermalconductivity with minimal amount of binders. In some embodiments,binders and sorbents may be dry mixed to enhance thermal conductivity.

In some embodiments, thermal conductivity between the sorbent materialand the thermistor may be improved by maximizing the contact surfacearea between a thermistor and the printed sorbent material. The thinprinted sorbent bed may be used so that both sorbent material heatconduction and sorbent/thermistor interface heat conduction can beimproved. In some embodiments, calendaring with a mild pressure may alsobe used to further enhance the thermal conductivity.

The configurations discussed below are generally described as having asingle sorbent material. In some embodiments, a gas monitor may havemore than one sorbent material. For example, more than one sorbentmaterial may be placed at different positions on a substrate to provideadditional accuracy. Furthermore, in some embodiments the gas monitormay have more than one sorbent material of different types. For example,a first sorbent material may target a first gas and a second sorbentmaterial may target a second gas. The gas monitor can then monitor theconcentration of multiple gasses. In some embodiments with multiplesorbent materials, there may be a single reference material portion thatis used as a control for each of the sorbent materials. For example, oneor more reference materials may be a carbon nanotube based structure.The reference material may be the same or similar type of material asdescribed with reference to the sorbent materials above, for instance.In some embodiments, there may be additional reference materials thatare used to act as controls for one or more other sorbent materials. Insome embodiments, the reference material may be the same material as thesorbent material, but that hasn't been activated, or may be encapsulatedsuch that it does not interact with the monitored environment.

Other gas monitors may use different sorbents materials to target othergasses. For example, other sorbents may be reasonably selective throughboth thermal modulation and surface treatments to change the functionalgroups of the previously mentioned sorbent materials to adsorb carbonmonoxide, Benzene, formaldehyde, or other biologically reactive gasses.As an example, Benzene may be monitored in a manufacturing setting toensure a safe work environment and proper functioning of machinery. Forexample, Benzene may be selectively adsorbed using this kind ofapproach. As another example, Formaldehyde may be monitored to determineif a person has been smoking in a room.

In addition to measuring air quality and presence of gasses in indoorspaces, gas monitors based on the temperature change of sorbents may beused to monitor the air quality in cars, submarines, boats, outdoorspaces such as stadiums, inside manufacturing facilities or machines, orthe like. In some embodiments, the gas monitors as described herein maybe used to monitor a target gas for other relevant environments.

FIG. 1 is a diagram showing an example embodiment of a gas monitor 100.The gas monitor includes a sorbent material 110 and a reference material115. The temperature of sorbent material 110 and reference material 115may be measured by respective thermistors 120, 125. In some embodiments,the gas monitor 100 may have other temperature measurement devices. Aprocessing device 130 may receive signals from thermistors 120, 125 andbased on a difference between the temperature measurements, may make adetermination of the concentration of a target gas within air flow 150.

In some embodiments, the sorbent material 110 may be a sorbent printedon a substrate 101 that selectively adsorbs a target gas. In someembodiments, the sorbent material may have been combined with a binderand solvent, then printed to produce a porous structure with highsurface area to mass ratio. The high surface area may increase theeffect of adsorption on the temperature of the sorbent material 110. Insome embodiments, the sorbent material may be a powder, solid core, orother structure rather than a printed sorbent.

The reference material 115 may be similar structurally to the sorbentmaterial 110. For example, the reference material 115 may be printed ona substrate 101 in a same or similar manner as the sorbent material 110.For example, the reference material 115 may be printed using a similarbinder and solvent that was used to print the sorbent material 110.Furthermore, if the sorbent material 110 has other structures, thereference material 115 may have a similar structure or manufacturingprocess. In some embodiments, the reference material 115 may havesimilar thermal properties as the sorbent material 110. For example, thereference material 115 may have similar size, weight, thermalconductivity, and other properties as the sorbent material 110. However,compared to the sorbent material 110, the reference material 115 may notbe responsive to the target gas or other gasses that are likely to bepresent in air flow 150. In some embodiments, reference material 115 maybehave similarly to the sorbent material 110 in response to gasses otherthan the target gas. In some embodiments, the reference material 115 mayinclude polymer beads or metal/oxide particles.

In some embodiments, thermistor 120 and thermistor 125 may be similarcomponents. For example, the thermistors 120, 125 may have the same orsimilar structure. In some embodiments, thermistor 120 and thermistor125 may have different structures. Furthermore, while discussed asthermistors, in some embodiments, the thermistors may be replaced withother temperature sensing devices. In some embodiments, thermistors 120,125 may be disposed within the sorbent material 110 and referencematerial 115. For example, the thermistors 120 may extend within thesorbent material 110 to increase the accuracy or speed of temperaturemeasurements. The thermistors 120, 125 may be electronically coupled toa processing device 130. For example, thermistors 120, 125 may providean indication of temperature of the sorbent material 110 and referencematerial 115.

The processing device 130 may receive signals from thermistors 120, 125that indicate the temperature of the sorbent material 115 and referencematerial 110. The processing device may include one or more processorssuch as a microprocessor, central processing unit, or the like. In someembodiments the processing device 130 may be an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), adigital signal processor (DSP), network processor, or the like.Furthermore, the processing device may include one or more memorydevices such as a main memory, random access memory, or other computerreadable storage mediums.

In some embodiments, the thermistors 120, 125 may be electronicallycoupled to the processing device 130 in a manner to provide adifferential measurement of temperature of the sorbent material 110 andthe reference material 115. In some embodiments, the processing device130 may determine a temperature difference between the sorbent material110 and the reference material 115 based on different indications oftemperature from the thermistors 120, 125. In some embodiments, theprocessing device may use the temperature differential to determine achange in the concentration of a target gas. For example, the processingdevice 130 may determine that the concentration of the target gas hasincreased in view of a higher temperature indication from thermistor 120compared to the temperature indication from thermistor 125. Furthermore,the processing device 130 may determine that the concentration of thetarget gas has decreased in view of a lower temperature indication fromthe thermistor 120 compared to the temperature indication from thethermistor 125.

In some embodiments, the processing device 130 may provide a signal ofchanges to the concentration of the target gas to another system. Forexample, the processing device 130 may provide an indication to acontrol system, an alarm system, or another system of changes to theconcentration of the target gas. In some embodiments, the processingdevice 130 may be a remote system to the sorbent material 110 andreference material 115. For example, thermistors 120, 125 may provideindications of temperature of the sorbent material 110 and referencematerial 115 through a wired or wireless electronic connection to theprocessing device 130 at a remote location.

In some embodiments, the processing device 130 may determine a change inthe concentration of the target gas based on the determined temperaturedifferential between the sorbent material 110 and the reference material115. However, as discussed above, in some embodiments, a gas monitor maydetermine the absolute concentration of the target gas in air flow 150.

FIG. 2 is a diagram showing an example embodiment of a gas monitor 200.The gas monitor 200 may be similar to the gas monitor 100 described withreference to FIG. 1. For example, the gas monitor 200 may include asorbent material 210 with associated thermistor 220, and a referencematerial 215 with associated thermistor 225. Thermistors 220, 225 may beelectrically coupled to processing device 230 to provide an indicationof the temperature of the sorbent material 210 and reference material215.

In addition to the described portions of gas monitor 200 that aresimilar to those of gas monitor 100, the gas monitor 200 may includeheating elements 240, 245. The heating element 240 may be operativelycoupled to the sorbent material 210. Thus, the heating element 240 mayprovide heat to the sorbent material 210 and the heating element 245 mayprovide heat to the reference material 215. In some embodiments, theheating element 240 and the heating element 245 may be combined as asingle heating element. Furthermore, in some embodiments, the thermistor220 and heating element 240 may be the same element and the thermistor225 and the heating element 245 may be the same element. For example,the thermistor 220 may both change resistance to provide an indicationof temperature and be used as a heating element by providing currentacross the thermistor 220. The thermistor 225 may be used in a similarmanner to provide heat to the reference material. In some embodiments,heating elements 240, 245 may be resistive heating elements that provideheat to the sorbent material 210 and the reference material 215 inresponse to a current across the heating elements.

The processing device 230 may be coupled to the heating element 240 andheating element 245 to control the heating elements. In someembodiments, the processing device 230 may provide current across theheating elements 210, 215 in a periodic manner to raise and lower thetemperature of the sorbent material 210 and reference material 215. Thesorbent material 210 and reference material 215 may then changetemperature in response to the provided heat. The reference material 210may increase and decrease temperature at a different rate than thesorbent material 210. For example, the sorbent material 210 may increaseand decrease temperature due to the change in heat provided from heatingelement 240, but may also adsorb and release the target gas based on thechange in temperature. Accordingly, the thermistor 220 associated withthe sorbent material 210 and the thermistor 225 associated with thereference material 215 may provide a differential indication oftemperature of the sorbent material 210 and reference material 215. Insome embodiments, the gas monitor 200 may determine an amount of currentused to heat a sorbent to a reference temperature. The amount of currentused may indicate the amount of the target gas on the sorbent 210.Accordingly, the processing device 230 may determine a concentration ofthe target gas in the environment based on the amount of current used.

Based on the differential temperature measurements provided by thethermistors 220, 225, the processing device 230 may determine anabsolute concentration of the target gas in air flow 250. For example,the processing device may compare the temperature indications from thethermistor 225 and 220 to a set of calibrated values for the temperatureof the sorbent material 210 at particular temperatures andconcentrations of the target gas to determine an absolute measurement ofthe concentration of the target gas.

While the components of gas monitor 200 have been shown in a particularconfiguration, in other embodiments, the components may be configureddifferently in different embodiments. For example, while shown in threelayers, in some embodiments, the thermistors 220, 225, sorbent material210, reference material 215, and heating elements 240, 245 may beconfigured differently. Furthermore, the positions of the sorbentmaterial 210 with respect to the reference material 215 may bedifferent. In some embodiments, the thermistors 220, 225 and heatingelements 240, 245 may be on opposite sides of the sorbent material 210and reference material 215. Furthermore, in some embodiments, thethermistors 220, 225, sorbent material 210, reference material 215, andheating elements 240, 245 may be coplanar. For example, the heatingelement 240 may be on one side of a sorbent material 210 and thethermistor 220 may be on the other side in a printed plane. In someembodiments, other configurations may be used, for example, componentsof the gas monitor 200 may be arranged in a concentric configuration,stacked configuration, in any number of layers, or in otherconfigurations wherein the components of the gas monitor operate asdescribed with reference to the gas monitor 200.

FIGS. 3A and 3B depict graphs showing the relationship ofcharacteristics of an example carbon based sorbent to the adsorption ofCO₂ by the sorbent. FIG. 3A shows the relationship between the quantityof CO₂ adsorbed by a sorbent and the change in the heat of adsorptionfor the system. As shown in FIG. 3A, as the sorbent absorbs more CO₂,the heat of adsorption in the sorbent material approaches the latentheat of vaporization of CO₂ in the environment. FIG. 3B shows therelationship between the concentration of the CO₂ in the monitoredenvironment and the amount of CO₂ on a carbon sorbent. The graphincludes a first curve showing the relationship at 20° C. and a secondcurve showing the relationship at 30° C. As shown in the graph, at alower ambient temperature (the 20° C. curve), the amount of CO₂ on thecarbon sorbent is higher for a given CO₂ concentration in the air.

FIG. 4 is a graph showing a differential temperature response of asorbent material and reference material when exposed to changing CO₂concentrations, according to an embodiment. FIG. 5 is a graph showing adifferential temperature response of a sorbent material and referencematerial when exposed to changing CO₂ concentrations, according to anembodiment. FIG. 5 shows the response to changing CO₂ concentrations ata different time scale. As can be seen in both FIG. 4 and FIG. 5, as theCO₂ concentration is increased in the environment that a gas monitor isexposed to, the temperature differential between the sorbent materialand reference material correspondingly changes. Notably, as CO₂ isexposed to the gas monitor, the temperature differential is increasedand as CO₂ is removed from the environment, the temperature differentialis decreased.

Because the sorbent temperature only provides an indication of the heatof adsorption, it acts as a derivative operation and cannot alonequantify the absolute CO₂ concentration. This effect can be seen inFIGS. 4 and 5, in which the thermistor response appears as a derivativeof the CO₂ concentration. In order to determine the absoluteconcentration of CO₂, a gas monitor may integrate the thermistordifferential output over time.

In some embodiments, the differential temperature indications may beintegrated to determine aggregate changes over time. The differentialtemperature indications may be integrated using a processing device orintegrating circuit. In some embodiments, the differential temperatureindications may be integrated in combination with using a modified4-wire resistance measurement scheme. For example, a Wheatstone bridgeconfiguration may be used to reduce the effects of noise and drift togenerate an integrated signal.

FIG. 6 is a graph showing adsorption loading of CO₂ gas on a sorbent asa function of temperature and gas concentration. The graph showschanging in temperature based on increasing and decreasing theconcentration of CO₂ at different temperatures. As shown in FIG. 2, agas monitor may change the temperature of a sorbent material andreference material to effect the adsorption and release of CO₂ from thesorbent material.

In some embodiments, based on the known measurements of a target gas anda sorbent temperature change as described with reference to FIG. 6, aprocessing device may apply a probe current to create a voltage acrossthe target resistor to apply a heat source to a sorbent material andreference material. In some embodiments, a separate circuit may employ ahigh impedance amplifier to measure the voltage. In some embodiments,the probe current may simultaneously heat the sorbent in order tovolatilize adsorbed target gasses. For example, the processing devicemay modulate a probe current in order to alternatively heat and cool thesorbent and to create a periodic signal that will enable absolute CO₂concentration measurement. Absolute CO₂ concentration quantification bymodulation of the sorbent temperature is possible because the CO₂ has arelatively fixed vapor pressure and heat of adsorption. Modulation ofthe sorbent temperature correspondingly shifts a Langmuir Isotherm curveassociated with the sorbent and target gas. This may cause the sorbentto alternatively adsorb or release the target gas. Thus, the sorbentmaterial may be moved along the x-axis of FIG. 6. The concentration ofCO₂ dictates which curve in FIG. 6 is sampled. The resulting thermalresponse of the sorbent from the adsorption/desorption can be measuredvia the thermistor circuit.

FIG. 7 is a flow diagram depicting a method 700 of determiningconcentration of a target gas from a differential temperature reading,according to an embodiment. In some embodiments, the method 700 may beperformed by a gas monitor 100 as described with reference to FIG. 1.For example, the processing device 130 in FIG. 1 may perform theprocesses described with respect to method 700.

Beginning in block 710, a gas monitor may receive an indication of atemperature of a sorbent material exposed to a target gas in theenvironment. For example, the indication of the temperature of thesorbent material may be generated by providing a probe current to athermistor coupled to the sorbent material. The voltage measured acrossthe thermistor may provide an indication of the temperature of thesorbent material.

In block 720, a gas monitor may receive an indication of a temperatureof a reference material. For example, the indication of the temperatureof the reference material may be generated by providing a probe currentto a thermistor coupled to the reference material. The voltage measuredacross the thermistor may provide an indication of the temperature ofthe reference material. The reference material may have physical andthermal properties similar to those of the sorbent material. Forexample, the reference material may change temperature in a similarmanner in response to changed temperature in the environment. However,the reference material may be selected to not respond to the target gasor other gasses likely to be in the monitored environment. In someembodiments, the reference material may be the same as the sorbentmaterial, but may have an encapsulation layer to prevent chemicalinteraction with the environment. Therefore, the layer may be thinenough to be thermally transparent, but prevent response to a targetgas.

Moving on to block 730, the gas monitor may determine a differentialbetween the sorbent material and the reference material. In someembodiments, the gas monitor may convert the indications of temperaturereceived from the thermistors to corresponding temperature values. Insome embodiments, the differential may be a difference between voltagesreceived from the thermistors and may not be converted to correspondingtemperatures. In some embodiments, the differential may be determined byproviding a filter circuit to generate a signal representing thedifferential between the sorbent material and the reference material.

In block 740, the gas monitor may determine a change in concentration ofa target gas based on the temperature differential. In some embodiments,the gas monitor may determine an amount of change in concentration basedon the amount of different between the temperature readings. The gasmonitor may also determine a length of time of the temperaturedifferential to determine an absolute change in the concentration. Forexample, the gas monitor may integrate the differential to determine theabsolute change in the concentration over a length of time.

FIG. 8 is a flow diagram depicting a method 800 of determiningconcentration of a target gas from a differential temperature reading,according to an embodiment. In some embodiments, the method 800 may beperformed by a gas monitor 200 as described with reference to FIG. 2.For example, the processing device 230 in FIG. 2 may perform theprocesses described with respect to method 800.

Beginning in block 810, a gas monitor may periodically drive a heatingelement operatively coupled to a sorbent material. In some embodiments,the heating element may be a resistive heating element that is driven bycurrent provided by a processing device. The heating element may bedriven with a sinusoidal waveform, a square waveform, or anotherwaveform. In some embodiments, the heating element may be pulsed with adriving current to raise and lower the temperature. In some embodiments,the period of driving the heating element may be on a scale of about 0.1Hz to about 100 Hz. In some embodiments, other driving signals may beprovided to the heating element. In some embodiments, the heatingelement may also be coupled to a reference material or the gas monitormay also periodically drive a heating element associated with areference material.

In block 820, the gas monitor may periodically sample an indication of atemperature differential between a sorbent material and a referencematerial. In some embodiments, the indication of the temperaturedifferential of the sorbent material and the reference material may beprovided by thermistors coupled to the sorbent material and thereference material. In some embodiments, the temperature differentialmay be sampled at a higher rate than the heating element is driven toprovide data about the change in temperature at different points in thedriving cycle.

In block 830, the gas monitor may identify an isotherm curve for thetarget gas of the sorbent material based on the periodic sampling of thetemperature differential. For example, as discussed with reference toFIG. 6, multiple isotherm curves show how the amount of CO₂ on thesorbent material relates to the temperature of the material at differentCO₂ concentrations in the monitored environment. The reference materialwould not change the relationship of its temperature in response to theheating element. Accordingly, by changing the temperature of the sorbentmaterial by driving the heating element and measuring the temperaturechange between the sorbent material and the reference material, the gasmonitor may determine an absolute concentration of CO₂ in the monitoredenvironment. For example, the gas monitor may use a lookup table orarray of data to determine an isotherm curve from the temperaturedifferential. Other target gasses that are monitored using othersorbents may have similar isotherm curves that can be used in a similarmonitor by the gas monitor.

In block 840, the gas monitor determines an absolute concentration ofthe target gas in the monitored environment from the isotherm curve. Forexample, after the isotherm curve is determined by comparing thetemperature of the sorbent material to the temperature of the referencematerial, the gas monitor may determine a corresponding absoluteconcentration of the target gas from the curve. In some embodiments, thegas monitor may then provide the concentration to another system,provide an alert or alarm if certain conditions are met, or otherwiseuse the data to track the concentration of the target gas in themonitored environment.

FIG. 9 is a flow diagram depicting a method 900 of manufacturing a gasmonitor, according to an embodiment. For example, the method 900 shownin FIG. 9 may be used to manufacture the gas monitors 100, 200 describedwith reference to FIGS. 1 and 2. Beginning in block 910, a firstthermistor and a second thermistor are operatively coupled to asubstrate. In some embodiments, the first and second thermistor may becoupled in a position, such as a cavity, in the substrate that isprepared for the thermistors. For example, the substrate may have beenetched to provide a receiving area for the thermistors. In someembodiments, the thermistors may be formed on the substrate with aprinting process, may be soldered to the substrate, or may otherwise becoupled to the substrate.

In block 920, a sorbent material is deposited on the substrate coupledto the first thermistor. In some embodiments, the sorbent material maybe combined with a binder and a solvent and printed in a manner to beoperatively coupled to the first thermistor. Accordingly, the firstthermistor may provide an accurate indication of the temperature of thesorbent material. In some embodiments, the sorbent material may beformed from a powder or other structure rather than a printed structure.

In block 930, a reference material is deposited on the substrate coupledto the second thermistor. In some embodiments, the reference materialmay be a selected such that it does not adsorb the target gas. Thereference material may be combined with a binder and a solvent andprinted in a manner to be operatively coupled to the second thermistor.Accordingly, the second thermistor may provide an accurate indication ofthe temperature of the reference material. In some embodiments, thereference material and the sorbent material are printed using the sameor similar binders and solvents to improve consistency of thetemperature change due to ambient temperature in the environment orheating elements. In some embodiments, the reference material may beformed from a powder or other structure rather than a printed structure.

In block 940, the thermistors may be coupled to a processing device. Forexample, the processing device may be one of processing device 130, 230discussed with respect to FIG. 1. In some embodiments, the thermistorsmay be coupled to the processing device using traces on the surface ofthe substrate. In some embodiments, the processing device may be coupledto the thermistors using one or more leads separate from the substrate.The processing device may then drive and receive signals from thethermistors to determine the concentration of a target gas in amonitored environment.

FIG. 10 a diagram showing an example embodiment of a gas monitoringsystem 1000. In some embodiments, the gas monitoring system 1000includes a control system 1040, a ventilation system 1050, and a numberof gas monitors 1010, 1020, 1030. For example, the gas monitors 1010,1020, 1030 may be similar to those described with reference to FIGS. 1and 2. In some embodiments, there may be fewer or additional gasmonitors 1010, 1020, 1030 than shown in FIG. 10. Gas monitors 1010,1020, 1030 may each measure concentration of one or more gasses. In someembodiments, the gas monitors 1010, 1020, 1030 monitor the same gas orgasses. For example, each of the gas monitors 1010, 1020, 1030 maymonitor CO₂ at different locations within a facility. In someembodiments, gas monitors 1010, 1020, 1030 may measure the concentrationof different gasses.

In some embodiments, the gas monitors 1010, 1020, 1030 may communicatewith a control system 1040 over a network 1060. For example, the networkmay be a wired or wireless network including one of a local areanetwork, an intranet, an extranet, the Internet, or another network. Thecontrol system 1040 may receive indications of temperature of sorbentand reference materials from the gas sensors and determine aconcentration of target gasses for each gas monitor 1010, 1020, 1030based on those indications. In some embodiments, the gas monitors 1010,1020, 1030 transmit indications or measurements of the concentration ofgas at the gas monitors 1010, 1020, 1030. Additionally, the gas monitors1010, 1020, 1030 may transmit alerts or alarms based on one or morethresholds regarding the concentration of gasses at the gas monitors1010, 1020, 1030.

In some embodiments, the control system 1040 may log data received fromthe gas monitors 1010, 1020, 1030. The control system 1040 may alsodetermine one or more actions to take based on the data received fromthe gas monitors 1010, 1020, 1030. For example, if one or more of thegas monitors 1010, 1020, 1030 provide data indicating a gasconcentration above or below a threshold level, the control system 1040may activate one or more other systems to respond to the change. As anexample, if the control system 1040 determines that a concentration ofCO₂ indicated by one or more of the gas monitors 1010, 1020, 1030 isnearing or above a threshold level, the control system 1040 may providecommands to a ventilation system 1050 to increase the level ofventilation. In some embodiments, the control system 1040 may be coupledto additional systems to address additional gas concentrations. Forexample, if particular gas concentrations are detected, the controlsystem 1040 may activate systems to decontaminate one or more rooms,monitor one or more rooms, or otherwise address potential adverseconsequences due to the concentration of gas in a facility.

Although described with respect to gas monitoring of a facility, in someembodiments, the gas monitoring system 1000 may be deployed in otherenvironments. For example, the gas monitoring system 1000 may bedeployed in a car to manage interior air quality, in outdoor venues, orin other applications to monitor and log or respond to the concentrationlevels of one or more target gasses.

Various operations are described as multiple discrete operations, inturn, in a manner that is most helpful in understanding the presentdisclosure, however, the order of description may not be construed toimply that these operations are necessarily order dependent. Inparticular, these operations need not be performed in the order ofpresentation.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present disclosure. Thus, the specific details set forth are merelyexemplary. Particular embodiments may vary from these exemplary detailsand still be contemplated to be within the scope of the presentdisclosure.

Additionally, some embodiments may be practiced in distributed computingenvironments where the machine-readable medium is stored on and orexecuted by more than one computer system. In addition, the informationtransferred between computer systems may either be pulled or pushedacross the communication medium connecting the computer systems.

Embodiments of the claimed subject matter include, but are not limitedto, various operations described herein. These operations may beperformed by hardware components, software, firmware, or a combinationthereof.

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittent oralternating manner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Moreover, use of the term “an embodiment” or “one embodiment” or“an implementation” or “one implementation” throughout is not intendedto mean the same embodiment or implementation unless described as such.Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. asused herein are meant as labels to distinguish among different elementsand may not necessarily have an ordinal meaning according to theirnumerical designation.

It will be appreciated that variants of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be combined intomany other different systems or applications. Various presentlyunforeseen or unanticipated alternatives, modifications, variations, orimprovements therein may be subsequently made by those skilled in theart which are also intended to be encompassed by the following claims.The claims may encompass embodiments in hardware, software, or acombination thereof.

What is claimed is:
 1. A gas monitor comprising: a sorbent material thatselectively adsorbs a target gas based on a concentration of the targetgas in a monitored environment, wherein the sorbent material isconfigured to passively absorb the target gas without heat beingprovided to the sorbent material; a reference material that is notresponsive to the target gas; a first thermistor disposed within thesorbent material and a second thermistor disposed within the referencematerial, the first thermistor to provide a first indication of a firsttemperature of the sorbent material and the second thermistor to providea second indication of a second temperature of the reference material;and a processing device to determine a concentration of the target gasbased at least in part on a differential measurement between the firsttemperature and the second temperature.
 2. The gas monitor of claim 1,further comprising: a first heating element to provide heat to thesorbent material; and a second heating element to provide heat to thereference material, wherein the first heating element and the secondheating element are to provide periodic heating to the sorbent materialand the reference material to determine an absolute value of theconcentration of the target gas or calibrate the gas monitor.
 3. The gasmonitor of claim 2, wherein the processing device is to determine theconcentration of the target gas based on mapping the differentialmeasurements to an isotherm curve for the target gas and sorbent.
 4. Thegas monitor of claim 1, wherein the sorbent material comprises amicroporous or nano-porous carbon material and the target gas is carbondioxide.
 5. The gas monitor of claim 1, wherein the target gas is one ofcarbon dioxide, carbon monoxide, benzene, or Formaldehyde.
 6. The gasmonitor of claim 1, wherein the sorbent material comprises a printedsorbent ink with a binder.
 7. The gas monitor of claim 1, furthercomprising a chamber allowing gas flow along the sorbent material andthe reference material.
 8. The gas monitor of claim 1, furthercomprising: a second sorbent material that selectively adsorbs a secondtarget gas; and a third thermistor disposed within the second sorbentmaterial, wherein the processing device is further to determine aconcentration of the second target gas based at least in part on anoutput of the third thermistor.
 9. A method comprising: receiving anindication of a temperature of a sorbent, wherein the sorbent changestemperature based on a heat of adsorption of a target gas, wherein thesorbent is configured to passively absorb the target gas without heatbeing provided to the sorbent; and determining, based at least in parton the indication of the temperature of the sorbent, a concentration ofthe target gas in a monitored environment in contact with the sorbent.10. The method of claim 9, further comprising: receiving a secondindication of a second temperature of a reference material associatedwith the sorbent, wherein determining the concentration of the sorbentcomprises calculating a differential between the indication of thetemperature of the sorbent and the second indication of the secondtemperature of the reference material.
 11. The method of claim 9,further comprising: periodically driving a heating element operativelycoupled to the sorbent, wherein determining the concentration of thesorbent comprises determining an absolute value of the concentrationbased on a difference in the indication of the temperature of thesorbent relative to a second temperature due to the heating element. 12.The method of claim 11, further comprising determining the secondtemperature based on a second indication of the second temperaturereceived from a thermistor coupled to a reference material that does notrespond to the target gas.
 13. The method of claim 11 further comprisingcomparing changes in the difference to an isotherm curve for the sorbentin the presence of the target gas.
 14. The method of claim 11, whereinperiodically driving the heating element comprising driving the heatingelement at a rate greater 0.1 Hz and less than 100 Hz.
 15. The method ofclaim 9, wherein the target gas is one of carbon dioxide, carbonmonoxide, benzene, or Formaldehyde.
 16. A method comprising: coupling afirst thermistor and a second thermistor to a substrate; depositing asorbent material on the substrate to couple the sorbent material to thefirst thermistor, wherein the sorbent material selectively adsorbs atarget gas based on a concentration of the target gas in a monitoredenvironment, and wherein the sorbent material is configured to passivelyabsorb the target gas without heat being provided to the sorbentmaterial; depositing a reference material on the substrate to couple thereference material to the second substrate; and coupling the firstthermistor and the second thermistor to a processing device.
 17. Themethod of claim 16, wherein the method further comprises: providing afirst heating element coupled to the sorbent material; and providing asecond heating element coupled to the reference material, wherein thefirst heating element and the second heating element are to provideperiodic heating to the sorbent material and the reference material todetermine an absolute value of the concentration of the target gas. 18.The method of claim 16, wherein depositing the sorbent materialcomprises printing the sorbent material mixed with at least one of abinder or a solvent.
 19. The method of claim 16, wherein depositing thesorbent material further comprises depositing the sorbent material witha porous glassy solid binder.
 20. The method of claim 16, whereindepositing the sorbent material further comprises depositing a sorbentink comprising the sorbent material, a styrene acrylic-based polymerlatex, and a solvent.